Lanxess AG said Tuesday that it would join with private-equity firm Advent International Corp. to buy Royal DSM NV’s DSM, +2.64% engineering-materials business for around 3.7 billion euros ($3.99 billion) and create a new venture for high-performance plastics.

The announcement came after the Netherlands-based DSM said earlier Tuesday that it would merge with Swiss fragrance company Firmenich SA to create a company with a EUR25 billion market cap.

The DSM engineering-materials business, which would become part of the joint venture, represents sales of around EUR1.5 billion and operating-profit margin of around 20%, Lanxess LXS, +2.09% said.

The German chemical company said it would also transfer its own high-performance materials business into the joint venture, and receive EUR1.1 billion initially and a stake of up to 40% in the new business.

Lanxess said it would use the proceeds to reduce debt and launch a EUR300 million share-buyback program.

Advent will hold at least 60% in the joint venture, Lanxess said.

The deal is expected to close in the first half of 2023, subject to approval by authorities, Lanxess said.

Write to Ed Frankl at edward.frankl@dowjones.com